Internal finned tube



July 30, 1968 J. F. PEARSON INTERNAL FINNED TUBE Filed Feb. 21, 1966INVENTOR JOHN F. PEARSON Z 2 g 2 MM M3 5 e 7 5' 9' era/#2 EXTRACTED.-'/00, 000

TIG 6 ATTORNEYS United States Patent 3,394,736 INTERNAL FINNED TUBE JohnF. Pearson, Jackson, Mich., assignor to Acme Industries, Inc., Jackson,Mi'ch., a corporation of Delaware Filed Feb. 21, 1966, Ser. No. 528,9194 Claims. (Cl. 138--38) ABSTRACT OF THE DISCLOSURE A tube type heatexchanger wherein internal fins are located within a tube, the finshaving a close interference fit with the inner wall of the tube, andimproved heat exchanging characteristics being obtained by the spiralconfiguration of the fins, and the presence of notches in the finsadjacent the tube inner wall to permit heat exchanger medium flowbetween adjacent chambers within the tube defined by the fins.

The present invention is an improvement in the heat exchangerconstruction described in the assignees United States Patent 2,929,408,issued Mar. 22, 1960.

In the aforementioned patent the advantages derived by forming a tubularheat exchanger from a tube and an internal fin member, wherein aninterference fit between the internal fin member and the tubular memberis produced, are discussed in detail. In the aforementioned patent a finmember having a plurality of radially extending fins is placed within atubular member and the tubular member is swaged, drawn, or otherwisereduced in diameter to form an intimate interference fit engagementbetween the ends of the fins and the inner surface of the tubularmember. The intimate interconnection between the ends of the fins andthe inner surface of the tubular member provides a most effective heattransfer connection between the fins and tubular member substantiallyincreasing the heat transfer ability of this type of heat exchanger overinternal fin heat exchangers wherein the fins are connected to thetubular member by mechanical means, brazing or soldering. Tubular heatexchangers of this type are commonly used in refrigeration systemswherein a refrigerant passes through the tubular member through the flowpaths defined by the fins, and the medium to be cooled is exposed to theexterior of the tubular members.

One of the problems encountered in any heat exchanger wherein a heatreceiving or emitting medium is flowing through a heat exchanger arisesfrom flow patterns occurring within the heat exchanger. Of course, themedium must be in engagement with a heat transfer member of the heatexchanger in order to effectively transfer heat. As the medium flowsthrough a tubular heat exchanger, it tends to channel in a flow pathhaving the least resistance to flow, and the mechanical surfaces of theheat exchanger are often not directly exposed to that portion of theheat transfer medium which is most capable of receiving or transmittingheat. To counteract this type of problem, agitators, baffles, and othermeans are often employed within heat exchangers to agitate the medium sothat new portions and molecules of the heat transfer medium arecontinually engaging the mechanical heat transfer surfaces of the heatexchanger.

In a heat exchanger in accord with the invention, wherein a finnedmember is located within a tubular member, such flow paths of therefrigerant through the passages defined by the fins and the tubularmember also develop which decrease the efficiency of the heat exchangingcharacteristics. Due to surface films existing on the fin surfaces andtube inner surface, it is p ssible for refrigerant to rapidly flowthrough the heat exchanger without coming in direct contact with eithera fin or the tube 3,394,736 Patented July 30, 1968 and, thus, portionsof the refrigerant flowing through the heat exchanger are noteffectively subjected to the mechanical surfaces of the heat exchangerand the efliciency thereof is adversely affected.

The present invention pertains to modifications made to the finconstruction disclosed and claimed in United States Patent 2,929,408wherein improved circulation of the refrigerant within the heatexchanger is provided. The innovations of the present invention haveincreased the heat exchanging capabilities of the heat exchanger tube ofthe aforementioned patent as much as one hundred percent under someconditions. In the present invention, the fin member is spiraledrelative to its longitudinal axis. Preferably, such spiraling is in theorder of approximately a rotation of a fin relative to the fin memberaxis for each linear foot of the fin member. Such spiraling of the finsimparts a rotational movement to the heat exchange medium, usually arefrigerant, about the axis or core of the finned member, andcentrifugal forces acting upon the molecules of the heat exchange mediumtend to throw the heavier molecules radially outward into engagementwith the inner surface of the tubular member. In a refrigerant suchheavier molecules would be the colder molecules and, thus, place suchmolecules in direct contact with the inner surface of the tubularmember. Also, such rotational movement of the heat exchange medium, ascaused by the spiraling of the fins, produces an internal movementwithin the heat exchange medium which scrubs the medium against thesurfaces of the fins, as well as against the inner surface of thetubular member.

Another feature of the invention which contributes toward improving theheat exchanging characteristics of an internal finned tubular heatexchanger lies in the provision of forming openings at axially spacedlocations in the fins immediately adjacent the inner surface of thetubular member. Such openings establish communication between the flowpaths defined in the tubular member and the fins and equalize theconditions of the heat exchange medium within the tubular member Withinthe various fiow paths thereof. By locating such openings adjacent theinner surface of the tubular member, movement of the heat exchangemedium through the opening is encouraged due to the centrifugal forcesacting upon the medium. Preferably, such openings constitute notcheshaving a concave, cylindrical segment surface, and the notches have aradial depth defined in a finned member substantially less than theradial dimension of the fin as not to significantly weaken the fin. Inthat the fin member is subjected to compressive forces as the tubularmember is reduced in diameter during the achievement of the interferencefit between the tubular member and the fin member, the notches must beso positioned and related to the fin member as to not adversely affectthe uniting of the fin member and the tubular member.

As the refrigerant flowing through an internal fin heat exchanger of thetype of the invention will often be in both a liquid and gaseous state,the provision of the openings adjacent the inner surface of the tubularmember prevents unequal ratios of refrigerant and gas from existing inthe various flow paths of the heat exchanger as defined by the fins and,thus, the heat absorbing characteristics of the heat exchanger willremain consistent throughout the circumferential dimension of thetubular member.

It is, therefore, an object of the invention to provide a heat exchangertube having internal fins wherein improved heat transfer medium flowthrough the tube is accomplished.

Another object of the invention is to provide a heat exchanger tube ofthe internal fin type wherein the fins are in an interference fitrelationship with a tubular member and wherein the fins are spiraledwith respect to the longitudinal axis of the tubular member.

Another object of the invention is to provide a heat exchanger tube ofthe internal fin type wherein the fins are spiraled with respect to theaxis of the tubular member whereby centrifugal forces are imposed uponthe heat transfer medium and openings are defined in the fins to permitcommunication between the fiow paths of the tube.

Yet another object of the invention is to provide a heat exchanger tubeof the internal fin type wherein the fins are in an interference fitrelationship with a tubular member, and wherein spiraling of the fins isprovided and communication is established between adjacent flow pathsdefined in the tube adjacent the inner surface of the tubular member,and wherein such spiraling of the fins and provision of such interfincommunication does not adversely affect the interference fit uniting thefinned member and the tubular member.

These and other objects of the invention arising from the details andrelationships of the components of an embodiment thereof will beapparent from the following description and accompanying drawingwherein:

FIG. 1 is a perspective view of a fin member constructed in accord withthe invention,

FIG. 2 is an elevational, diametrical, sectional view of a heatexchanger tube constructed in accord with the invention, the finnedmember and the tubular member being fully assembled and the finnedmember being shown in elevation for purposes of illustration,

FIG. 3 is an elevational, sectional view taken along section III-III ofFIG. 2,

FIG. 4 is an enlarged, detail, sectional view of the interconnection ofa fin member with the inner surface of the tubular member,

FIG. 5 is an enlarged, detail view of one of the notches defined in afin terminating edge, and

FIG. 6 is a graph illustrating the improvements derived from theincorporation of the inventive concepts of the invention.

In the drawings the tubular member is indicated at 10. Such tubularmember is of a cylindrical configuration having a cylindrical outersurface 12 and an inner cylindrical surface 14. The tubular member isformed of a relatively soft metal having a high coefiiciency of thermalconductivity. Copper or aluminum are usually employed, although copperis preferable and is most widely used.

A finned member 16 is located within the tubular member 10 and, in theillustrated embodiment, comprises a core portion 18, FIG. 3, defining anaxially extending axis, and a plurality of fins are integral with thecore portion 18 and radially extend therefrom. The finned member 16 isformed by an extrusion process and is formed of a metal also having ahigh coefiiciency of thermal conductivity. The material of the finnedmember is of a greater hardness than the material of the tubular memberin order to permit the proper interference fit to be accomplished. Inpractice, the tubular member is normally formed of copper and the finnedmember is formed of an alloy having a greater hardness than that ofcopper, such as one of the aluminum alloys 63S-T5, 63S-T6, or 63S-T2,properly aged.

The fins 20 of the finned member are preferably related to the core 18such that the fins are not in diametrical relation to each other. In thedisclosed embodiment, five fins 20 are illustrated. The fins are,preferably, of a reduced cross-sectional thickness adjacent the core 18as compared to the cross-sectional thickness of the fins adjacent theirassociated radial terminating edge 22. This gradual increase in the massof the fins in accord with the proximity to the tubular member innersurface 14 makes the most effective use of the material of the finnedmember as the flow of heat through the fins is greatest adjacent theedges 22. Also, by reducing the thickness of the fins 20 adjacent thecore portion 18, the maximum area of flow path between the fins can beachieved. Increased heat transfer between the fins and tubular member isalso obtained by the presence of a T-shaped section on the fins adjacentthe edges 22. The T-shaped cross section increases the circumferentiallength of the edges 22 over that which would exist if the Tconfiguration were not present and thereby increases the area of contactbetween the assembled tubular member 10 and finned member 16.

To provide the improvements mentioned above, with regard to the flow ofthe heat transfer medium through the heat exchange tube, the finnedmember 16 is spiraled with respect to its longitudinal axis. Thus, eachfin 20 is spiraled in the longitudinal direction about the axis of thecore 18. In practice, it has been found that a spiral between 174 andfor each linear foot of the finned member produces advantageous results.In the actual practice of the invention, the finned members are extrudedwith a 174 spiral, which means that the fin 20a, for instance, FIG. 1,will spiral 174 in the counterclockwise direction for each linear footof the finned member. The spiraling of each of the fins 20 is of anidentical nature in a common direction with respect to the axis of thefinned member.

The spiraling of the fins 20 of the finned member is of such a nature asnot to weaken the structural characteristics of the finned member, whichare necessary to withstand the forces imposed thereon by theinterference fit between the finned member and the tubular member.

In order to permit communication between the flow paths defined in theheat exchange tube by the fins 20, the fins are each notched adjacenttheir terminating edge 22 at axially spaced locations. The spacing ofthe notches 24 will be apparent from FIGS. 1 and 2. In one embodiment ofthe invention, the notches nearest an end of the finned member will beplaced approximately three and one-half inches from the end andthereafter the notches will be placed at eight inch intervals along theaxial length of the finned member. To simplify manufacturing techniquesthe notches 24 in each fin member are, preferably, located at identicalaxial locations, and the notches are usually formed by a punchingoperation. In the preferred embodiment, the notches 24 are defined by aconcave, arcuate surface 26 constituting a cylindrical segment surfacewhich intersects the associated terminating edge at spaced axiallocations 28 and 30, FIG. 5. Preferably, the surface 26 is of athree-quarter inch radius and the notches have a radial depth into theassociated fin from the terminating edge 22 of one-eighth of an inch. Asthe aforementioned dimensions of the notches 24 are defined in fins 20having a radial dimension no less than one-quarter of an inch, it willbe appreciated that the notches 24 only extend into the associated finabout half of the radial dimension of the fin on the smaller finnedmember sizes and, thus, the fins and finned member are not undulyweakened by the presence of the notches. With the preferred dimensionalrelationship set forth above, the axial dimension separating thelocations 28 and 30' of a common notch, wherein the notch intersects theassociated fin terminating edge, is more than five or six times thedimension of the radial depth of the notch in the fin. This relationshipappears to provide a most advantageous communication between the flowpaths of the heat exchanger tube without producing an excessive pressuredrop within the heat exchanger tube.

To assemble the heat exchanger tube, the finned member 16 is locatedwithin the tubular member 10. The dimension of the finned member in atransverse crosssectional form is such that the circumference defined bythe fin terminating edges 22 is slightly less than the internalcircumference of the tube and the finned member may be readily insertedwithin the tubular member. Upon the finned member being properly axiallylocated within the tubular member, the tubular member is radiallycontracted by a drawing operation, whereby the inner surface 14 of thetubular member is brought into an interference fit relationship with theterminating edges 22 of the fins. This relationship is best appreciatedfrom FIG. 4. As

will be seen from FIG. 4, the interference fit between the tubularmember and the fins causes the material of the tubular member tointimately engage the fin terminating edge and the lateral portionthereof adjacent the terminating edge and forms an effective heatconducting connection between the internal fins and the tubular member.The fact that the fins 20 are not diametrically related permits theinterference fit to be accomplished without fracturing the finned memberor the tubular member and, as the notches 24 have not unduly weakenedthe finned member, the aforedescribed interconnection between the finnedmember and the tubular member can take place without damage to thefinned member.

It will be appreciated that the aforedescribed assembly of the finnedmember 16 and the tubular member will locate the notches 24 immediatelyadjacent the inner surface of the tubular member, whereby thecommunication between the flow paths Within the tubular member definedby the fins 20 will be immediately adjacent the inner surface 14 of thetubular member.

In practice, a common means for utilizing the heat exchanger tubes ofthe invention is to place a plurality of these tubes in parallelrelationship extending through a casingin which water or brine may becirculated. As refrigerant passes through the tubes, the refrigeranttherein absorbs heat from the surrounding water or brine and reduces thetemperature thereof. The water or brine is circulated through the casingand is pumped to cooling coils or the like and used for cooling. It isthe usual practice that the refrigerant travels through the heatexchanger tubes in such a manner that the tubes are arranged in aplurality of passes whereby the refrigerant passes through variousseries of heat exchange tubes to permit the refrigerant to absorb themaximum amount of heat fromthe water or brine.

When the refrigerant is initially introduced into the heat exchangertubes, it is usually in the form of both a liquid and a gas. Therefrigerant typically may initially consist of 80% liquid refrigerantand 20% gas, as it leaves the expansion valve and is introduced into theheat exchanger tubes. After several passes through the heat exchangercasing, the refrigerant will consist entirely of a gas. The velocity ofthe refrigerant flowing through the heat exchanger tubes in a typicalheat exchanger installation may lie between seven hundred feet perminute and fifteen hundred or more feet per minute. As the liquid andthe gaseous refrigerant rapidly moves through the heat exchanger tubes,the spiral of the fins will rotate the medium and produce centrifugalforces within the medium. Thus, the heavier molecules of refrigerant,such as the colder gaseous molecules and the liquid molecules will tendto be disposed adjacent the inner surface 14 of the tubular member 10and, thus, be in the best position for absorbing heat from the tubularmember wall. In addition to the optimum refrigerant distributionobtained by the spiraling or rotational movement of the refrigerant tothe heat exchanger tube, the existence of the notches or openings 24permits the refrigerant to be uniformly distributed throughout thecross-sectional configuration of the heat exchanger tubes. Thus, no oneflow path through the tubes will contain a disproportionate percentageof liquid or gas refrigerant with respect to another flow path.Equalized flow characteristics through the flow paths of a heatexchanger tube are, therefore, produced and the pressure dropcharacteristics of the tube are maintained at a minimum so as not toadversely affect the saturation temperature.

The graph in FIG. 6 compares the heat-exchanging characteristics of aninternal finned heat exchanger tube of the nonspiraled, nonnotched typeof United States Patent 2,929,408, but having T-sectioned terminatingfin edges with the improved construction in accord with the invention.The curve 28 represents the heat-exchanging results of a tubeconstructed in accord with Patent 2,929,408 wherein the fins of thefinned member are not spiraled but are of a linear configuration andthere is no communication between the flow paths defined in the tube bythe fins of the finned member. The curve 30 represents the heat exchangecharacteristics obtained by a heat exchanger tube in accord with theinvention. The vertically disposed numerals on the graph represent thesaturation temperature i ndegrees Fahrenheit of the refrigerantcorresponding to the pressure at the refrigerant outlet as therefrigerant leaves the test cooler casing. The horizontally disposednumerals indicate the amount of heat the refrigerant within the heatexchanger tubes has extracted from the cooler within the test coolercasing and is indicated in B.t.u.s/hour divided by 100,000. Theconditions of temperature of test Water to be cooled introduced into thetest cooler, rate of Water flow, number of tubes and passes, and otherconditions necessary to permit comparison of the heat-exchangingcharacteristics of the two types of internal finned tubes weremaintained in deriving curves 28 and 30.

At an evaporation temperature of 36 F. the B.t.u./hour/ 100,000extracted by the unimproved heat exchanger tube is 5.25, while the tubein accord with the invention extracts 8.00, an improvement of 1.52. Athigher evaporation temperatures the improvement is even greater. At 40%F. the B.t.u./hour/ 100,000 of the unimproved tube is 2, while theimproved tube extracts 4.2 for an increase of 2.02.

It will, therefore be appreciated from the above that the spiraling andnotching of the fins substantially inrproves the heat-exchangingcapacity of this type of internal finned heat exchanger. While thespiraling of the fins does slightly increase the refrigerant pressuredrop within the tubular member, the increase in improved heat transfermore than offsets the disadvantages occurring from the increasedpressure drop.

It is appreciated that various modifications to the inventive conceptmay appear to those skilled in the art without departing from the spiritand scope thereof, and it is intended that the invention be defined onlyby the following claims.

What is claimed is:

1. A heat exchanger tube for conducting a refrigerant comprising, incombination:

(a) an outer tubular member of relatively soft metal having an innercylindrical surface,

(b) an internal, integral fin member disposed within said tubularmember, said fin member having a solid central core portion and beingformed by an extrusi'on process and of a metal having a greater hardnessthan the material of said tubular member,

(c) a plurality of fins defined on said fin member radially extendingfrom said central portion, said fins being of elongated configurationand each being radially defined by a terminating edge, said finterminating edges having an interference fit with said tubular memberinner surface providing an intimate heat-conductive engagement betweensaid fins and said tubular member, said fins being spirally disposed ina common direction about the longitudinal axis of said fin member, and

(d) a plurality of notches defined in each of said fins, said notchesbeing axially spaced along each of said fins, intersecting theterminating edge of the associated fin and of a radial dimension lessthan the radial dimension of the associated fin whereby said notchesdefine openings adjacent the tubular member inner surface establishingcommunication between adjacent refrigerant flow paths defined in saidtubular member by said fins.

2. In a heat exchanger tube as in claim 1 wherein:

(a) said fins each have a cross section increasing in thickness from apoint adjacent said central core portion outwardly to the associatedterminating edge,

(b) said fins being disposed out of diametrical relation to each other,and

(c) a preformed T-shaped transverse cross section defined on each ofsaid fins adjacent the terminating edge thereof, the upright 0f theT-shaped cross section being defined by the associated fin and thecrossed portion thereof defining said terminating edge and the maximumtransverse dimension of the associated fin.

3. In a heat exchanger tube as in claim 2 wherein:

(a) said fins of said fin member spiral in a common direction about saidfin member longitudinal axis at a rate whereby the pitch of said fins isapproximately one complete spiral about said axis for every two linearfeet of said fin member.

4. In a heat exchanger tube as in claim 3 wherein:

(a) the axial distance separating said spaced locations defined by theintersection of a notch concave surface with the associated finterminating edge is at least five times the radial depth of said notchin the associated fin.

References Cited UNITED STATES PATENTS ROBERT A. OLEARY, PrimaryExaminer.

15 T. W. STREULE, Assistant Examiner.

